Donna and Trip, maybe Paul can call Carnival, RCI, and others and check for the correct answer? To say "I thought" and "from what I've read" when the OP has already talked to Carnival and RCI does not seem like the best answers. Paul is really good at tracking down the right answers with the cruise lines; this seems like a good question for him to ask. Althought "my recollection" is the same as yours, it would be great to have it verified.
edited to add:
Looks like the OP also asked on another forum to get similar replies. However, he called the pier and learned the following:
I spoke with the pier this morning. RCCl will not allow anyone to book at the pier less than 24 hours in advance. Carnival however will allow bookings at the pier. The cruises I were interested in just happened to be fully booked with no cancellations, otherwise they would have been glad to accept bookings at the pier..
As for the legal requirements, the passenger manifest must be transmitted 15 minutes prior to sailing; however, the requirement is changing to 60 minutes. Here is the link to the Federal Register:
I don't know where this 72 hour rumour got started; it would be great to get the correct information more widely disseminated.